Arc furnaces

ABSTRACT

An electric arc furnace adapted to be powered by direct current includes a bottom electrode of one polarity (positive) mounted in the vessel for contact with the charge and three electrodes of negative polarity depending into the vessel from above. The arcs from the electrodes migrate inwardly which is the opposite of the case with a.c. thus reducing erosion of the furnace walls by concentrating the heat source in the centre. The bottom electrode design is such as to enable the furnace to be tapped whilst maintaining a reservoir of metal for the succeeding charge.

United States Patent 1191 Bowman ARC FURNACES [75] Inventor: BrianBowman, Geneva,

Switzerland [73] Assigneez British Steel Corporation, London,

England [22] Filed: Jan. 5, 1973 [21] Appl. No.: 321,273

[30] Foreign Application Priority Data Jan. 18, 1972 1 Great Britain2421/72 [52] US. Cl 13/12, 13/16 [51] Int. Cl. F27d 11/10 [58] Field ofSearch 13/9, l2, IO

[56] References Cited UNITED STATES PATENTS 2,789,152 4/1957 Ham et al.13/9 X 1451 Jan. 29, 1974 2,448,886 9/1948 Hopkins 13/9 2,789,150 4/1957Clough at al. 13/9 x 3,180,916 4/1965 Menegoz et al. 13/9 PrimaryExaminerRoy N, Envall, Jr. Attorney, Agent, or Firm-Bacon & Thomas 57ABSTRACT An electric arc furnace adapted to be powered by direct currentincludes a bottom electrode of one polarity (positive) mounted in thevessel for contact with the charge and three electrodes of negativepolarity depending into the vessel from above. The arcs from theelectrodes migrate inwardly which is the opposite of the case with ac.thus reducing erosion of the furnace walls by concentrating the heatsource in the centre. The bottom electrode design is such as to enablethe furnace to be tapped whilst maintaining a reservoir of metal for thesucceeding charge.

7 Claims, 6 Drawing Figures PATENTEB JAN 2 9 IFIE'H SHEET 2 OF 2 FIG.4b.

ARC FURNACES This invention relates to are furnaces and moreparticularly relates to d.c. arc furnaces for refining metal e.g.,melting and refining steel scrap.

The present invention comprises a direct-arc furnace adapted to bepowered by direct current and including at least one electrode of onepolarity mounted in the refining vessel for contact with the charge anda plurality of electrodes of the opposite polarity dependent into thevessel above the charge.

Preferably, the latter electrodes are symmetrically disposed around acircular path concentric with the vessel walls and are all connected tothe negative source of polarity, functioning as cathodes. The vesselelectrode (anode) may be constituted by a reservoir of the metal refinedlocated in the base or hearth of the vessel with an elongated fillet orplug extending from it for contact with the positive source of supply.During melting or refining the reservoir will melt but contact with thebus bar will always be maintained through the solid/liquid interfacethus formed in this electrode.

The disposition and design of the vessel electrode is such that themolten reservoir is not completely evacuated during tapping so that thenecessary contact is available for the succeeding charge.

A d.c powered arc furnace in accordance with this invention results in amore stable and uniform arc than is the case with ac. since thedependent electrodes operate continuously as cathodes and notalternately as cathode and anode, accordingly consumption of the(graphite) electrodes is-less. Operation is also quieter than with acfurnaces of comparable rating. More sig nificant however is thereduction in erosion of the refractory lined walls of the furnace byreason of the arcs from the multiple electrodes migrating inwardlytowards the centre of the furnace instead of migrating outwardly towardsthe walls as is the case with ac. Thus substantial savings in operatingcosts may be achieved.

In order that the invention may be fully understood one embodimentthereof will now be described, by way extending through a roof (notshown).. The power supply for the furnace is derived from astar/deltaconnected transformer 5, the secondary windings of whichseparately apply an appropriate phase to fullwave rectifier banks 6,7,8comprising silicon rectifiers.

The positive outputs from these banks are commoned and coupled viaa'lead or bus bar 9 to an electrode in the vessel which is in contactwith the melt, whilst the negative connections are separately coupledvia leads or bus bars 10, l 1, 12 to the electrodes 2,3,4 respectively.

With this form of coupling the arc voltage is increased by over its a.c.value whilst the current is decreased by this amount the same power andMVA obtains in each case. I

One of the more significant advantages of d.c. over a.c. in accordancewith this invention is, as mentioned above, the fact that whereas a.c.arcs from multiple electrodes migrate outwardly from the electrode tips(commonly known as blow-out) d.c. arcs migrate inwardly by reason of theelectro-magnetic field set up thus the power input to the melt isconcentrated in the centre of the furnace. This aspect is shown in FIGS.2a and 2b.

More particularly FIG. 2a shows the electrodes disposed above the moltenbath (13) in elevation from which it can be seen that the slag 14 on thesurface of the melt is to a large degree blown to the centre of thefurnace e.g., away from the refractory lining on the furnace walls thusreducing hot spot erosion arising from radiation/convection from theflames and/or slag at tack. This is illustrated in plan in FIG. 2b, theappropriate lobes of power dissipation by the arcs being identified bynumerals 15,16 and 17.

The three electrodes are symmetrically arranged around a circleconcentric with the walls of the furnace and, as compared with acoperation, the pitch circle diameter on which these electrodes lie ispreferably greater than the ac. case in order to ensure that scrap willbe adequately melted off the banks adjacent the walls of the vessel.

The vessel (positive) electrode is sited in its base or hearth andprovides a liquid/solid steel connection.

Referring now to FIG. 3 the electrode is constituted by a steelreservoir 18 and a fillet or elongated plug 19 the extremity of which iswater-cooled, contained in a specially designed receptacle 20 in thebase. This may in fact be a castable refractory, it being understoodthat in a practical furnace the whole of this base protrusion could wellbe housed in refractory brickwork. The

common bus bar 9 is secured to the remote end of a leg 21 of thisreceptacle 20. The plug, a solid steel member, is preformed with theelectrode itself, e.g., by a casting process, and machined to ensuregood contact with the bus bar.

The leg 21 extends in the same direction as the bus bar and the furnacetilts about an axis substantially parallel with these members.

With this arrangement then, when a hot metal and scrap charge isintroduced this makes contact with the vessel electrode and the chargeis melted in the normal way as with ac. Any melting of the metalreservoir in the vessel electrode is of no consequence since contact isstill maintained through the resulting solid/liquid interface. Inpractice, with the bath molten the temperature at the remote end of theplug 19 may be only of the order of 200l300 C so that the bus bar isunaffected by this contact and, by the same token, no impurities are fedback to contaminate the melt.

When the charge is ready for tapping the vessel is tilted as shown inFIG. 40 but, by reason of the disposition of the reservoir, it will beseen that it is not completely evacuated of molten metal. Thus, when theves se] is ready for charging again (FIG. 4b) a solid or liquid mass ofsteel is still within the reservoir so that contact may readily beestablished for the following heat.

Although this invention has been described with reference to theparticular embodiments illustrated it is to be understood that variousmodifications and alterations may be made without departing from thescope of this invention. For example the star/delta power inputarrangement has merely been illustrated as an expedient since existinga.c. transformers for are furnaces are conventionally of this type. Astar connected secondary could equally well be used in which case thed.c. voltages and currents would be at the same level as for an ac.furnace. Compared with a delta secondary then, the star secondary wouldnot reduce electrode consuption to the same level by reason of thehigher current but the arc length would be reduced because of thereduced voltage.

Moreover, as regards the rectification, a multiphase rectified supplyfor each electrode would be preferable to the single-phase system shownbut against this multiphase rectifiers tend to introduce harmonics intothe supply.

I claim:

1. A direct arc furnace comprising:

a refining vessel for receiving a steelmaking charge;

a source of d.c.;

at least one electrode mounted in the vessel for contact with thecharge;

a plurality of electrodes dependent into the vessel above the charge,the electrodes being symmetrically arranged around a circular path aboutthe vertical axis of the vessel; and

means for connecting the d.c. source between the vessel electrode on theone hand and the plurality of dependent electrodes on the other.

2. A furnace according to claim 1, wherein the dependent electrodes areconnected as cathodes to the negative supply of the d.c. source.

3. A furnace according to claim 2, wherein the vessel electrode isconstituted by a reservoir of the refined steel located in a well in thebase or hearth of the vessel, and including an enclosed rod of metalextending from the reservoir for connection to the positive supply ofthe d.c. source.

4. A furnace according to claim 3, wherein the reservoir is bounded by abowl-shaped depression whereby, when the vessel is tilted for tapping,the reservoir is never completely evacuated.

5. A furnace according to claim 4, wherein the rod lies in a directionparallel with the axis about which the furnace is tilted.

6. A furnace according to claim 5, wherein said d.c. source isconstituted by a multi-phase rectifier bank whereby to provide amulti-phase rectified supply for each electrode.

7. A direct arc furnace comprising:

a refining vessel for receiving a steelmaking charge;

a source of'd.c.;

a plurality of massive electrodes dependent into the vessel above thecharge and symmetrically arranged around a circular path about thevertical axis of the vessel;

means for connecting the negative supply of the d.c.

source in common to said plurality of electrodes;

at least one electrode mounted in the vessel for contact with thecharge; and

means for connecting said one electrode to the positive supply of thed.c. source and wherein said one electrode is constituted by a reservoirof the refined steel located in a bowlshaped depression in the wall orhearth of the-vessel whereby, when the vessel is tilted for tapping,

the reservoir is never completely evacuated.

1. A direct arc furnace comprising: a refining vessel for receiving asteelmaking charge; a source of d.c.; at least one electrode mounted inthe vessel for contact with the charge; a plurality of electrodesdependent into the vessel above the charge, the electrodes beingsymmetrically arranged around a circular path about the vertical axis ofthe vessel; and means for connecting the d.c. source between the vesselelectrode on the one hand and the plurality of dependent electrodes onthe other.
 2. A furnace according to claim 1, wherein the dependentelectrodes are connected as cathodes to the negative supply of the d.c.source.
 3. A furnace according to claim 2, wherein the vessel electrodeis constituted by a reservoir of the refined steel located in a well inthe base or hearth of the vessel, and including an enclosed rod of metalextending from the reservoir for connection to the positive supply ofthe d.c. source.
 4. A furnace according to claim 3, wherein thereservoir is bounded by a bowl-shaped depression whereby, when thevessel is tilted for tapping, the reservoir is never completelyevacuated.
 5. A furnace according to claim 4, wherein the rod lies in adirection parallel with the axis about which the furnace is tilted.
 6. Afurnace according to claim 5, wherein said d.c. source is constituted bya multi-phase rectifier bank whereby to provide a multi-phase rectifiedsupply for each electrode.
 7. A direct arc furnace comprising: arefining vessel for receiving a steelmaking charge; a source of d.c.; aplurality of massive electrodes dependent into the vessel above thecharge and symmetrically arranged around a circular path about thevertical axis of the vessel; means for connecting the negative supply ofthe d.c. source in common to said plurality of electrodes; at least oneelectrode mounted in the vessel for contact with the charge; and meansfor connecting said one electrode to the positive supply of the d.c.source and wherein said one electrode is constituted by a reservoir ofthe refined steel located in a bowl-shaped depression in the wall orhearth of the vessel whereby, when the vessel is tilted for tapping, thereservoir is never completely evacuated.